1. Field of the Invention:
This invention relates to a thermal printing head which is used for example to print on thermosensitive paper or to cause ink transfer from a thermal transfer ribbon or film onto printing paper. More particularly, the present invention relates to improvements in a thermal printing head of the type wherein a connector board is overlapped on a head circuit board for connection to external circuits.
2. Description of the Prior Art:
As is well known, thermal printing heads are widely used in facsimile machines to print transmitted information on thermosensitive paper. The thermal printing head is also used in printers of the type wherein the ink of a transfer ink ribbon or film is thermally caused to be transferred onto printing paper.
There are various types of thermal printing heads which include line-type heads and matrix-type heads. The line-type thermal printing head has a row (line) of multiple heating dots, as disclosed for example in Japanese Patent Application Laid-open No. 63-151466 or No. 63-221055. The matrix-type thermal printing head has a multiplicity of heating dots arranged in a matrix, as disclosed for example in U.S. Pat. No. 3,855,448 to Hanagata et al.
The present invention is directed primarily but not exclusively to the line-type thermal printing head. To clarify the objects of the present invention, reference is now made to FIG. 10 which shows a typical line-type thermal printing head.
As shown in FIG. 10, the prior art thermal printing head comprises an elongate head circuit board 21 adhesively mounted on an elongate heat sink plate 30. The head circuit board comprises a substrate 22 which is formed with a longitudinal row (line) of multiple heating dots 23 positioned adjacent one longitudinal side of the head circuit board. The substrate 22 is also formed with a conductor pattern which includes a common electrode 24 and connection terminals 25 located adjacent the other longitudinal side of the circuit board in a comb-like arrangement. The common electrode 24 has both ends 24a located also adjacent the other longitudinal side of the circuit board to serve as connection terminals together with the other connection terminals 25.
The substrate 22 of the head circuit board 21 also carries a plurality of drive IC's 26. These drive IC's are connected to the heating dots 23 through individual electrodes (not shown) which form part of the conductor pattern. The heating dots are also connected to the common electrode 24.
The comb-like connection terminals 25 are divided into groups corresponding in number to the drive IC's 26. The comb-like connection terminals in each group are wired to the corresponding drive IC to serve respectively as a data input terminal (DI), a data output terminal (DO), a source voltage terminal (VDD), a strobe terminal (STR), a grounding terminal (GND) for that particular drive IC. In other words, the respective drive IC's each have a required number of comb-like connection terminals of their own.
On the heat sink plate 30 is further mounted a strip-like flexible connector board 31 which is reinforced by a backing 32. In an assembled state, the backing 32 disposed clear of the head circuit board 21 to directly rest on the heat sink plate. The flexible board 31 has a front marginal portion 31a projecting beyond the backing to partially overlap the head circuit board 21. Though not shown, the underside of the flexible board is formed with connection terminals in corresponding relation to the connection terminals 24a, 25 of the head circuit board, and the backing 32 supports a connector having pins in conduction with the connection terminals of the flexible board.
An elongate presser cover 35 is arranged above the connector board 31, and fixed to the heat sink plate 30 by means of mounting screws 37. For this purpose, the presser cover and the connector board 31 (the backing 32 as well) are respectively formed with mounting holes 35a, 33 through which the mounting screws 37 are inserted into engagement with threaded holes 30a of the heat sink plate. The underside of the presser cover is provided with an elastic rod 36 for pressing the front marginal portion 31a into contact with the head circuit board 21 when the mounting screws are tightened up.
With the arrangement described above, since the connection terminals 24a, 25 of the head circuit board 21 are distributed over the entire length of the circuit board, the flexible connector board 31 together with the backing 32 must be correspondingly elongated. For instance, when printing on JIS-A4 size papers (JIS: Japanese Industrial Standards), the connector board must have a length of about 21 cm at least. When printing on JIS-B4 size papers, the connector board must have a length of 26 cm at least. Obviously, such elongation of the connector board and the backing leads to material waste in addition to causing weight increase.
More importantly, to ensure uniform terminal contact between the connector board and the head circuit board, it is necessary to use a sufficient number of mounting screws 37 to flexible connector board, which also necessitates provision of corresponding number of mounting holes 33, 35a, and threaded holes 30a. Further, all of the mounting screws 37 must be tightened up in a well controlled manner. All these prolong the time required for manufacturing the thermal printing head, thereby leading to production cost increase.
Moreover, the prior art thermal printing head has a serious problem of bending at the time of printing operation, thereby deteriorating the printing quality. The reason for such bending is as follows.
Generally, the heat sink plate 30 and the presser cover 35 are equally made of aluminum because this material is light in weight and yet easily formed into any desired shape. Therefore, these two parts have the same coefficient of linear expansion. However, the heat sink plate 30 receives heat immediately from the head circuit board 21, whereas the presser cover 35 receives heat indirectly through the mounting screws 37 with a time lag. Thus, at the time of initiating the actuation of the heating dots 23 or abruptly changing the actuating voltage for the heating dots, the heat transmitting time lag leads to a difference in the degree of longitudinal expansion between the heat sink plate and the presser cover at least before reaching a steady state.
According to the prior art arrangement shown in FIG. 10, the entire length of the presser cover 35 is fixedly mounted to the heat sink plate 30 by the mounting screws 37. Thus, when the presser cover and the heat sink plate are longitudinally expanded to different degrees, the thermal printing head as a whole bends longitudinally as a result of the so-called "bimetal phenomenon".